1. Field of the Invention
The present invention relates to certain improvements in the preparation of ammonia from a hydrogen- and nitrogen-containing ammonia synthesis gas.
More particularly, this invention concerns synthesis of ammonia at high production capacity in at least two ammonia synthesis reactors arranged in parallel synthesis loops with one combined compression step of synthesis recycle gas.
2. Description of the Related Art
Synthesis of ammonia conventionally is performed by passing a process stream of synthesis gas, containing nitrogen and hydrogen in essentially stoichiometric amounts, to the formation of ammonia through a catalyst arranged in one or more beds in a reactor.
Thermodynamically, the reaction of hydrogen and nitrogen to ammonia requires the reaction to be performed at elevated temperature and pressure, usually above 100 atmospheres and at temperatures between 300.degree. and 600.degree. C. At the above process conditions, the equilibrium concentration of ammonia in a stoichiometric synthesis gas composition is below 20% by volume in the synthesis gas stream from the reactor. The stream has to be recycled to the reactor, together with fresh synthesis gas, in order to provide a reasonably sufficient ammonia product yield.
Prior to recycling, the stream is cooled to separate ammonia from unreacted hydrogen, nitrogen, and inert diluents present in the synthesis gas. A fraction of the recycle gas is purged to vent inerts. The stripped gas is then passed to a compression stage by which it is recycled to the reaction.
Expenses for compressing and recycling synthesis gas are important factors in the economy of ammonia production in general, and in particular when production capacities of existing ammonia synthesis loops have to be increased.
Accordingly, the need exists for ammonia production processes having reduced expenses, such as those associated with compressing and recycling synthesis gas. In addition, the need exists for economical methods of increasing the production capacities of existing ammonia synthesis plants.